Iphone 在UITabBarController上添加UIPickerView

我试图从屏幕底部（选项卡栏顶部）播放UIPickerView幻灯片，但似乎无法显示。动画的实际代码来自苹果的一个示例代码项目（DateCell）。我从选项卡栏控制器下的第一个视图控制器（FirstViewController.m）调用此代码

- (IBAction)showModePicker:(id)sender {
if (self.modePicker.superview == nil) {
    [self.view.window addSubview:self.modePicker];

    // size up the picker view to our screen and compute the start/end frame origin for our slide up animation
    //
    // compute the start frame
    CGRect screenRect = [[UIScreen mainScreen] applicationFrame];
    CGSize pickerSize = [self.modePicker sizeThatFits:CGSizeZero];
    CGRect startRect = CGRectMake(0.0, screenRect.origin.y + screenRect.size.height, pickerSize.width, pickerSize.height);
    self.modePicker.frame = startRect;

    // compute the end frame
    CGRect pickerRect = CGRectMake(0.0, screenRect.origin.y + screenRect.size.height - pickerSize.height, pickerSize.width, pickerSize.height);

    // start the slide up animation
    [UIView beginAnimations:nil context:NULL];
    [UIView setAnimationDuration:0.3];

    // we need to perform some post operations after the animation is complete
    [UIView setAnimationDelegate:self];

    self.modePicker.frame = pickerRect;

    // shrink the vertical size to make room for the picker
    CGRect newFrame = self.view.frame;
    newFrame.size.height -= self.modePicker.frame.size.height;
    self.view.frame = newFrame;
    [UIView commitAnimations];

    // add the "Done" button to the nav bar
    self.navigationItem.rightBarButtonItem = self.doneButton;
}}

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每当此操作通过位于UINavigationBar（都在FirstViewController下）中的uiBarButtonim触发时，都不会发生任何事情。有人能给我一些建议吗？

我发现我忽略了一个非常重要的事实，即UIPickerView也在Interface Builder中定义过。。。完成此操作并连接插座后，一切正常